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1.
We use a total of 839,369 PcP, PKPab, PKPbc, PKPdf, PKKPab, and PKKPbc residual travel times from [Bull. Seism. Soc. Am. 88 (1998) 722] grouped in 29,837 summary rays to constrain lateral variation in the depth to the core-mantle boundary (CMB). We assumed a homogeneous outer core, and the data were corrected for mantle structure and inner-core anisotropy. Inversions of separate data sets yield amplitude variations of up to 5 km for PcP, PKPab, PKPbc, and PKKP and 13 km for PKPdf. This is larger than the CMB undulations inferred in geodetic studies and, moreover, the PcP results are not readily consistent with the inferences from PKP and PKKP. Although the source-receiver ambiguity for the core-refracted phases can explain some of it, this discrepancy suggest that the travel-time residuals cannot be explained by topography alone. The wavespeed perturbations in the tomographic model used for the mantle corrections might be too small to fully account for the trade off between volumetric heterogeneity and CMB topography. In a second experiment we therefore re-applied corrections for mantle structure outside a basal 290 km-thick layer and inverted all data jointly for both CMB topography and volumetric heterogeneity within this layer. The resultant CMB model can explain PcP, PKP, and PKKP residuals and has approximately 0.2 km excess core ellipticity, which is in good agreement with inferences from free core nutation observations. Joint inversion yields a peak-to-peak amplitude of CMB topography of about 3 km, and the inversion yields velocity variations of ±5% in the basal layer. The latter suggests a strong trade-off between topography and volumetric heterogeneity, but uncertainty analyses suggest that the variation in core radius can be resolved. The spherical averages of all inverted topographic models suggest that the data are best fit if the actual CMB radius is 1.5 km less than in the Earth reference model used (i.e. the average outer core radius would be 3478 km).  相似文献   

2.
The derivation of P and S velocities at the core-mantle boundary (CMB) from long-period diffracted waves by the use of the simple ray-theoretical formulav CMB=r c /p (v CMB=velocity at the CMB;r c =core radius;p=ray parameter) yields apparent velocity values which differ from the true velocities. Using a dominant period of about 20 sec for calculating theoretical seismograms, we found a linear relation between the apparent velocity and the average velocity in a transition zone at the base of the mantle with fixed velocity on top.The ray parameters determined from long-period earthquake data are found to be 4.540±0.035 and 8.427±0.072 sec/deg for Pdiff and Sdiff, respectively. These values yield apparent velocities of 13.378±0.103 for P and 7.207±0.062 km/sec for S waves. By means of the theoretical relation between apparent and average velocity and under the assumption of linear variation of velocity with depth, one can invert the apparent velocities into true CMB velocities of 13.736±0.170 and 7.320±0.124 km/sec. These results imply positive velocity gradients at the base of the mantle and hence no significant departures from adiabaticity and homogeneity.Contribution No. 211 of the Geophysical Institute, University of Karlsruhe.  相似文献   

3.
Precursor and coda portions of short-period PcP waves (reflected P wave from the core-mantle boundary, CMB) recorded at J-array stations in Japan were analyzed in order to extract weak scattered signals originating from small-scale heterogeneities in the lowermost mantle beneath northeastern China. Two nuclear explosions at Lop Nor in China detonated on 21 May 1992 (Mb=6.5) and 8 June 1996 (Mb=5.9) were used for our analysis.Three-dimensional grids above the CMB were defined in the area around the PcP bounce points beneath northeastern China to calculate theoretical travel times of scattered waves which propagate from the sources to each grid point and arrive at each station based on the IASP91 model. Subsequently the waveforms were aligned with respect to the theoretical travel times and the semblance (an amplitude dependent measure of coherency) was calculated for each grid point. In order to obtain a more accurate travel time correction, we applied a cross correlation method to PcP waveforms in order to reduce picking error of the PcP onset time. A cross convolution method was also applied so that the two events could be analyzed simultaneously without using unstable deconvolutions.We could identify regions with relative high semblance values in semblance contour maps at about 200 and 375 km above the CMB. Stacking waveforms with respect to the theoretical travel times for the grid points with relative high semblance values indicate coherent wavelets originating at those grid points, that is, they correspond to scattered waves originating from small-scale heterogeneities in the lowermost mantle. Our results indicate the existence of small-scale scattering objects in the D″ layer, especially in the depth range of 200 and 375 km above the CMB beneath northeastern China. Considering recent tomographic images of high velocity anomalies in this area, these scattering objects could be fragments of old oceanic crusts which have subducted through the lower mantle and have accumulated in the D″ layer beneath northeastern China.  相似文献   

4.
Amplitude ratio of 30 short-period conspicuous P5KP and PKPab phases from five intermediate depth or deep events in Fiji-Tonga recorded at European stations around 150° distance shows a mean value two to three times the ratio of the synthetic amplitudes obtained by the normal-mode theory (and ak135 model) or by full-wave theory (and PREM). There is a large variance in the results, also observed in five amplitude ratios from one event in Argentina observed at temporary stations in China around 156°. Global recordings of three major deep earthquakes in Fiji, Bonin, and Western Brazil observed at ASAR, WRA, and ZRNK arrays, at 59 North America stations and at six South Pole stations displayed conspicuous P4KP and PcP (or ScP) phases. The amplitude ratio values of P4KP vs P(S)cP are sometimes almost one order of magnitude larger than the corresponding values of the synthetics. In both cases, arrival times and slowness values (corrected for ellipticity and station elevation) at the distances up to 23° beyond the A cutoff point predicted by ray theory match both the synthetics, suggesting the observations are the AB branch of PmKP (m?=?4, 5) around 1 Hz. In disagreement to ray theory, no reliable BC branch is observed neither on the recordings nor on the normal-mode synthetics. The high amplitude ratio values cannot be explained by realistic perturbations of the velocity or attenuation values of the global models in the proximity of the core-to-mantle boundary (CMB). We speculate that the focusing effects and/or strong scattering most likely associated to some anomalous velocity areas of the lowermost mantle are responsible for that. The results suggest limitations of the previous evaluations of the short-period attenuation in the outer core from PmKP amplitudes (m?≥?3), irrespective of the fact that they are obtained by using ray theory, normal-mode or full-wave synthetics. Attempts to use PmKP arrival times in order to refine velocity structure in the proximity of CMB should be also regarded with care if the propagation times have been computed with ray theory.  相似文献   

5.
Quantifying the density contrasts of the Earth's inner core boundary(ICB) is crucial to understand core-mantle coupling and the generation of the geodynamo. The PKiKP/PcP amplitude ratio is commonly used to obtain the density contrast at the ICB, but its applications are limited by scattered observed data. In this study, we selected the PKiKP and PcP phases reflected at the same region of inner-core and core-mantle boundaries beneath Northeast Asia from different earthquakes for the first time, and the observations suggested that the PKiKP/PcP amplitude ratio is widely scattered. We also compared the PKiKP and PcP amplitudes, which demonstrated that the scatter cannot be attributed only to ICB anomalies but might also arise from raypath differences and heterogeneities throughout the crust and mantle. By fitting the observed PKiKP/PcP amplitude ratio, we obtained a density contrast of approximately 0.65 g cm~(-3) and a compressional velocity contrast of approximately 0.87 km s~(-1) at the ICB beneath Northeast Asia. The larger contrast values indicate the possible occurrence of local crystallization occurring at the inner core surface.  相似文献   

6.
Computing synthetic seismograms for media with localized heterogeneous regions can be performed using hybrid methods. Here, a combination of a finite-difference (FD) technique and a frequency-wavenumber (ω − k) filtering is applied to model wave reflection at different kinds of core-mantle boundary (CMB) topography. The FD method is only applied in the neighbourhood of the CMB, while the ω − k filter is used to continue the reflected wavefield to the Earth's surface. Synthetic SH-seismograms for ScS with a dominant frequency of 0.5 Hz are computed at epicentral distances from 44° to 69°. The topography varies in amplitude (maximum amplitude of 1.0–2.7 km) and in its wavenumber spectrum; it is either monochromatic (wavelengths from 55 to 270 km) or statistical (coloured noise). The seismograms for a CMB with topography are compared with those for a plane CMB. We observe that monochromatic topography with short wavelengths (less than 100 km) results in amplitude reduction and shorter travel times than in the case of a plane CMB, but no variations with epicentral distance appear, whereas greater wavelengths exhibit amplitude variations with distance as well as travel time residuals, which both correlate with the CMB topography. Statistical models show amplitude variations with epicentral distance, while the travel time residuals are very small (less than 0.1 s). All synthetics illustrate that wavefront healing occurs along the ray path from the CMB to the Earth's surface. While the seismograms at the CMB exhibit strong fluctuations, the fluctuations at the surface are smoothed and reduced. This demonstrates that it is necessary to use wave theoretical methods for computing synthetic seismograms for complicated structures at greater depth. It also follows that travel times are less sensitive to the structure than the amplitudes.  相似文献   

7.
Seismic studies of the lowermost mantle suggest that the core-mantle boundary (CMB) region is strongly laterally heterogeneous over both local and global scales. These heterogeneities are likely to be associated with significant lateral viscosity variations that may influence the shape of the long-wavelength non-hydrostatic geoid. In the present paper we investigate the effect of these lateral viscosity variations on the solution of the inverse problem known as the inferences of viscosity from the geoid. We find that the presence of lateral viscosity variations in the CMB region can significantly improve the percentage fit of the predicted data with observations (from 42 to 70% in case of free-air gravity) while the basic characterisics of the mantle viscosity model, namely the viscosity increase with depth and the rate of layering, remain more or less the same as in the case of the best-fitting radially symmetric viscosity models. Assuming that viscosity is laterally dependent in the CMB region, and radially dependent elsewhere, we determine the largescale features of the viscosity structure in the lowermost mantle. The viscosity pattern found for the CMB region shows a high density of hotspots above the regions of higher-than-average viscosity. This result suggests an important role for petrological heterogeneities in the lowermost mantle, potentially associated with a post-perovskite phase transition. Another potential interpretation is that the lateral viscosity variations derived for the CMB region correspond in reality to lateral variations in the mechanical conditions at the CMB boundary or to large-scale undulations of a chemically distinct layer at the lowermost mantle.  相似文献   

8.
Global P-wave tomography: On the effect of various mantle and core phases   总被引：4，自引：0，他引：4  
In this work, many global tomographic inversions and resolution tests are carried out to investigate the influence of various mantle and core phase data from the International Seismological Center (ISC) data set on the determination of 3D velocity structure of the Earth's interior. Our results show that, when only the direct P data are used, the resolution is good for most of the mantle except for the oceanic regions down to about 1000 km depth and for most of the D″ layer, and PP rays can provide a better constraint on the structure down to the middle mantle, in particular for the upper mantle under the oceans. PcP can enhance the ray sampling of the middle and lower mantle around the Pacific rim and Europe, while Pdiff can help improve the spatial resolution in the lowermost mantle. The outer core phases (PKP, PKiKP and PKKP) can improve the resolution in the lowermost mantle of the southern hemisphere and under oceanic regions. When finer blocks or grid nodes are adopted to determine a high-resolution model, pP data are very useful for improving the upper mantle structure. The resulting model inferred from all phases not only displays the general features contained in the previous global tomographic models, but also reveals some new features. For example, the image of the Hawaiian mantle plume is improved notably over the previous studies. It is imaged as a continuous low velocity anomaly beneath the Hawaiian hotspot from the core-mantle boundary (CMB) to the surface, implying that the Hawaiian mantle plume indeed originates from the CMB. Low-velocity anomalies along some mid-oceanic ridges extend down to about 600 km depth. Our results suggested that later seismic phases are of great importance in better understanding the structure and dynamics of the Earth's interior.  相似文献   

9.
We describe a linear Bayesian inversion method to estimate the relevant petrophysical properties of the media forming a reflecting interface from the observations of amplitude variation with incidence angle. Three main steps characterize the proposed approach:
– information from borehole logs are statistically analysed to estimate the empirical models that describe the functional relationship between petrophysical (e.g. porosity, saturation, pressure or depth) and seismic variable(P and S velocities and density);
– the pure-mode (PP) reflection coefficient is parameterized in terms of the relevant petrophysical variables and is linearized in order to implement the linear inversion;
– the sought petrophysical parameters are estimated from the seismic reflected amplitudes by applying the linearized inversion where a priori information, data and model errors and solutions are described by probability density functions.
We test the method on synthetic and real data relative to reflections from a shale/gas-sand interface where the amplitude versus angle response, besides the lithological contrast, is mainly controlled by the saturation and porosity of the sand layer. The outcomes of the linearized inversion are almost identical to those obtained by a previously developed non-linear inversion method demonstrating the applicability of the linear inversion. It turns out that the gas-sand saturation in the range 0%–95% is a poorly resolved parameter while the porosity is the best resolved parameter. The issues of robustness and resolution of the inversion are discussed either through singular value decomposition analysis or the observation of the a posteriori probability density functions.
The linear inversion algorithm, compared with the previously developed non-linear method, reduces significantly the computation time allowing for more extensive applications.  相似文献   

10.
Proper stacking of three-dimensional seismic CDP-data generally requires the knowledge of normal moveout velocities in all source-receiver directions contributing to a CDP-gather. The azimuthal variation of the stacking velocities mainly depends on the dip of the seismic interfaces. For a single dipping plane a simple relation exists between the dip and the azimuthal variation of NMO-velocity. Varying strike and dip of subsequent reflectors, however, result in a complex dependency of the seismic parameters. Reliable information on the spatial distribution of the normal moveout (NMO)-velocity can be derived from a wavefront curvature estimation using a 3-D ray-tracing technique. These procedures require additional information, e.g. reflection time gradients or depth maps to show interval velocities between leading interfaces. Moreover, their application to an extended 3-D data volume is restricted by high costs. The need for a routine 3-D procedure resulted in a special data selection to create pseudo 2-D profiles and to apply existing velocity estimation routines to these profiles. At least three estimates in different directions are necessary to derive the full azimuthal velocity variation, characterized by the large and the small main axis and the orientation of the velocity ellipse. Errors are estimated by means of computer models. Stacking velocities obtained by mathematical routines (least-squares fit) and by seismic standard routines (NMO-correction and correlation) are compared. Finally, a general 3-D velocity procedure using cross-correlation of preliminarily NMO-corrected traces is proposed.  相似文献   

11.
The effects of variable speeds of spreading of submarine slides and slumps on near-field tsunami amplitudes are illustrated. It is shown that kinematic models of submarine slides and slumps must consider time variations in the spreading velocities, when these velocities are less than about 2cT, where is the long period tsunami velocity in ocean of constant depth h. For average spreading velocities greater than 2cT, kinematic models with assumed constant spreading velocities provide good approximation for the tsunami amplitudes above the source.  相似文献   

12.
The effects of unsuspected lateral variation in seismic velocities at depth on the interpretation of a seismic refraction profile are discussed with the aid of a numerical experiment. The results show that there will be bias in any velocity depth models derived by travel time analysis based on the assumption of horizontally layered media. This bias is examined for a lithospheric profile using both extremal and linearised travel time inversion. In addition quite mild subsurvace topography can have an appreciable effect on the amplitude distribution along the profile.  相似文献   

13.
Long period Rayleigh wave and Love wave dispersion data, particularly for oceanic areas, have not been simultaneously satisfied by an isotropic structure. In this paper available phase and group velocity data are inverted by a procedure which includes the effects of transverse anisotropy, anelastic dispersion, sphericity, and gravity. We assume that the surface wave data represents an azimuthal average of actual velocities. Thus, we can treat the mantle as transversely isotropic. The resulting models for average Earth, average ocean, and oceanic regions divided according to the age of the ocean floor, are quite different from previous results which ignore the above effects. The models show a low-velocity zone with age dependent anisotropy and velocities higher than derived in previous surface wave studies. The correspondence between the anisotropy variation with age and a physical model based on flow aligned olivine is suggestive. For most of the Earth SH > SV in the vicinity of the low-velocity zone. Neat the East Pacific Rise, however, SV > SH at depth, consistent with ascending flow. Anisotropy is as important as temperature in causing radial and lateral variations in velocity. The models have a high velocity nearly isotropic layer at the top of the mantle that thickens with age. This layer defines the LID, or seismic lithosphere. In the Pacific, the LID thickens with age to a maximum thickness of ～50 km. This thickness is comparable to the thickness of the elastic lithosphere. The LID thickness is thinner than derived using isotropic or pseudo-isotropic procedures. A new model for average Earth is obtained which includes a thin LID. This model extends the fit of a PREM, type model to shorter period surface waves.  相似文献   

14.
Elastic full waveform inversion of seismic reflection data represents a data‐driven form of analysis leading to quantification of sub‐surface parameters in depth. In previous studies attention has been given to P‐wave data recorded in the marine environment, using either acoustic or elastic inversion schemes. In this paper we exploit both P‐waves and mode‐converted S‐waves in the marine environment in the inversion for both P‐ and S‐wave velocities by using wide‐angle, multi‐component, ocean‐bottom cable seismic data. An elastic waveform inversion scheme operating in the time domain was used, allowing accurate modelling of the full wavefield, including the elastic amplitude variation with offset response of reflected arrivals and mode‐converted events. A series of one‐ and two‐dimensional synthetic examples are presented, demonstrating the ability to invert for and thereby to quantify both P‐ and S‐wave velocities for different velocity models. In particular, for more realistic low velocity models, including a typically soft seabed, an effective strategy for inversion is proposed to exploit both P‐ and mode‐converted PS‐waves. Whilst P‐wave events are exploited for inversion for P‐wave velocity, examples show the contribution of both P‐ and PS‐waves to the successful recovery of S‐wave velocity.  相似文献   

15.
PcP and PmKP travel times are computed for three simple or parametric Earth models, based on free-oscillation and travel-time data B1, PEM-A and HB1 and compared with PcP and PmKP travel times from different sources. This comparison is made only for the region above and below the core-mantle boundary and is of interest because of the current search for a standard Earth model. The comparison shows that only model B1 does not need a correction for its PcP travel times. For the PmKP travel times for the three models, corrections of the form Δt = a + bm were obtained. The models need the following corrections for b: ?1.3 for B1, 2.8 for HB1 and 0.6 for PEM-A. The corrections a are shown to be equal to the observed corrections for PcP at large epicentral distances. The inversions of free-oscillation data to obtain Earth models are most successful when body-wave phases that interact with the core are included.  相似文献   

16.
Foraminiferal tests are commonly found in tsunami deposits and provide evidence of transport of sea floor sediments, sometimes from source areas more than 100 m deep and several kilometers away. These data contribute to estimates of the physical properties of tsunami waves, such as their amplitude and period. The tractive force of tsunami waves is inversely proportional to the water depth at sediment source areas, whereas the horizontal sediment transport distance by tsunami waves is proportional to the wave period and amplitude. We derived formulas for the amplitudes and periods of tsunami waves as functions of water depth at the sediment source area and sediment transport distance based on foraminiferal assemblages in tsunami deposits. We applied these formulas to derive wave amplitudes and periods from data on tsunami deposits in previous studies. For some examples, estimated wave parameters were reasonable matches for the actual tsunamis, although other cases had improbably large values. Such inconsistencies probably reflect: (i) local amplification of tsunami waves by submarine topography, such as submarine canyons; and (ii) errors in estimated water depth at the sediment source area and sediment transport distance, which mainly derive from insufficient identification of foraminiferal tests.  相似文献   

17.
Variations of seismic interval velocities within the cable length cause anomalies in the stacking velocity analyses. Utilizing the approximation of rectilinear ray propagation, i.e. supposing that the velocity changes cause time delays only, it is shown that the stacking velocity anomalies are linearly related to the interval velocity variations. In particular, the stacking velocity anomaly is calculated when the interval velocity of an intermediate layer undergoes a stepwise variation. The amplitude of the anomaly increases with the ratio between horizon depth and cable length. From the forward model, a program for the inversion is derived in order to identify lateral changes of interval velocities from unsmoothed stacking velocity analyses. Some examples of the application of this technique to synthetic and real data are presented.  相似文献   

18.
The D″ region     
Two very different types of models are currently being proposed for D″, the lowest region of the earth's mantle: (a) those in which the P and S velocities vary smoothly down to the core-mantle boundary, without any extreme change in gradient; (b) those in which the velocity gradients decrease fairly abruptly at a height of 100 km or so above the core-mantle boundary, and maintain a value close to the critical gradient down to the boundary.Type (a) is represented by model UTD124A′ of Dziewonski and Gilbert (1972) and model B1 of Jordan and Anderson (1974). Both models are in good agreement with most travel time and free oscillation data. Their validity rests on the supposition, supported in part by theoretical studies, that data which suggest the presence of a low velocity zone in D″ result from distortion of seismic waves by the core-mantle boundary.On the other hand, slowness and amplitude data from short period P waves indicate a fairly rapid decrease in velocity gradient at a depth corresponding to an epicentral distance of about 92°, and it is very unlikely that these data can be interpreted as interface phenomena. The measured P and S times at distances beyond about 96° also indicate reduced velocities in D″. The suggestion that the measured velocities are in error as a result of interface effects is weakened by the fact that the results are apparently not wavelength-dependent.Type (b) is represented by model B2 of Jordan (1972), Bolt's (1972) model, and a new model designated as ANU2. All models have high density gradients indicative of inhomogeneity in the region. Model B2 fits the oscillation data reasonably well, but has an unjustifiably low S velocity at the core-mantle boundary. In Bolt's model the P and S velocities at the top of D″ are based on the models of Herrin et al. (1968) and Jeffreys (1939), whereas in ANU2 the values are taken from Hales and Herrin (1972) and Hales and Roberts (1970b). The velocities at the core-mantle boundary in Bolt's model and ANU2 are based on observations of “diffracted” P and S. Both of these models were designed to produce flattening of the P curve at about 92°. Both may require some modification in order to be compatible with free oscillation data.  相似文献   

19.
Tsunami generated by submarine slumps and slides are investigated in the near-field, using simple source models, which consider the effects of source finiteness and directivity. Five simple two-dimensional kinematic models of submarine slumps and slides are described mathematically as combinations of spreading constant or slopping uplift functions. Tsunami waveforms for these models are computed using linearized shallow water theory for constant water depth and transform method of solution (Laplace in time and Fourier in space). Results for tsunami waveforms and tsunami peak amplitudes are presented for selected model parameters, for a time window of the order of the source duration.The results show that, at the time when the source process is completed, for slides that spread rapidly (cR/cT≥20, where cR is the velocity of predominant spreading), the displacement of the free water surface above the source resembles the displacement of the ocean floor. As the velocity of spreading approaches the long wavelength tsunami velocity the tsunami waveform has progressively larger amplitude, and higher frequency content, in the direction of slide spreading. These large amplitudes are caused by wave focusing. For velocities of spreading smaller than the tsunami long wavelength velocity, the tsunami amplitudes in the direction of source propagation become small, but the high frequency (short) waves continue to be present. The large amplification for cR/cT1 is a near-field phenomenon, and at distances greater than several times the source dimension, the large amplitude and short wavelength pulse becomes dispersed.A comparison of peak tsunami amplitudes for five models plotted versus L/h (where L is characteristic length of the slide and h is the water depth) shows that for similar slide dimensions the peak tsunami amplitude is essentially model independent.  相似文献   

20.
Introduction The fluid outer core separates the solid inner core from the solid elastic mantle, and as a result, makes the free and forced movement of this mechanical system more complicated and profuse. As the elastic mantle, the free oscillations may occur within the Earths fluid outer core (FOC) due to excitation of a strong and deep earthquake (Crossley, 1975b; Friedlander, Siegmann, 1982; Shen, 1983; Friedlander, 1985). However, compared with the oscillations of the elastic mantle, i…  相似文献   

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